This invention relates to a prosthetic joint adapted to replace a human joint and, more specifically, to an improved prosthetic joint adapted to replace an interphalangeal joint of the human finger.
The first elongated bone (metacarpal) at the base of each finger is connected to a proximal phalangeal bone through the metacarpal-phalangeal (MCP) joint. The proximal phalangeal bone is connected to the middle phalangeal bone through the proximal interphalangeal (PIP) joint. The PIP joint can be flexed independently of the MCP or the distal interphalangeal (DIP) joint. This variable reciprocal motion, along with the opposability of the thumb, allows for grasping of objects and the performance of daily functions which are of critical importance to humans. Damage to the PIP joint through physical injury or disease can therefore be a severe physiological burden to inflicted humans.
Degenerative joint diseases: osteoarthritis (OA), post-traumatic arthritis, and rheumatoid arthritis (RA) of the PIP joint cause interminable pain and poor function of the finger. Patients who have mild symptoms often respond to rest, immobilization, nonsteroidal anti-inflammatory drugs or intra-articular injections of steroids. However, patients who have more severe forms of arthritis may require total joint replacement of the PIP joint.
The most common deformity in patients suffering from OA is narrowing of the articular cartilage of the PIP joint. OA is a hypertropic arthritis that results in degeneration of the articular cartilage which is often either primary or secondary to trauma or other conditions, and such degeneration of the cartilage causes movement of the joint to be painful. The usual solution is fusion of the joint which eliminates pain but renders the joint nonfunctional. This is particularly true in young patients with normal grip strength. The preferred solution is the installation of a prosthetic PIP joint, however, there has long been a need for a replacement joint which resurfaces the native joint, allows for normal joint motion, and withstands the joint reaction forces produced in patients with normal grip strength.
Various prosthetic joints have been developed to replace finger joints; however, most of these have involved the replacement of the MCP joint where the finger joins the hand. These prosthetic joints have typically involved some type of captive ball-and-socket arrangement for permitting a substantial range of pivoting movement in a vertical plane, while sometimes also permitting limited sideward displacement in an attempt to provide for motion which more closely correlates with the natural joint. Joints of this type are normally not suitable for replacing the proximal or distal interphalangeal joints of the finger because they typically possess a structure which is undesirably large and complex, i.e. interphalangeal joints desirably do not incorporate a capability for sideward or lateral pivoting.
Examples of finger joints which are designed specifically for replacing the human MCP joint are disclosed in U.S. Pat. Nos. 5,782,927, 4,731,087 and 4,231,121, with this latter patented joint being represented as also being suitable for use as a PIP or DIP joint. U.S. Pat. No. 4,725,280 discloses a joint which is designed specifically for use in replacing the human PIP joint or the DIP joint. More recent U.S. Pat. No. 5,728,163 (Mar. 17, 1998) discloses a MCP joint prosthesis which is alleged to also be useful as an interphalangeal joint prosthesis. Work was also done at The Mayo Clinic in the 1970""s directed to the PIP joint, and this work is reported in an article by Linscheid, R. L. et al, entitled xe2x80x9cProximal Interphalangeal Joints Arthroplasty with a Total Joint Designxe2x80x9d, Mayo Clin. Proc., 54, 227-240 (1979). Continuing work on this project was more recently reported in a further article by Linscheid et al. entitled xe2x80x9cDevelopment of a Surface Replacement Arthroplasty for Proximal Interphalangeal Jointsxe2x80x9d, The Journal of Hand Surgery, 22a, 286-298 (1997).
Although various of these PIP joints have had limited success, they have not gained wide acceptance throughout the medical profession. Accordingly, the search has continued for improved joints particularly suited for the replacement of the PIP joint that are implantable in such a manner as to facilitate its attachment to the phalanges while at the same time providing a vertical range of pivoting movement which closely approximates natural joint movement.
One important design consideration for PIP prosthetic joints is minimizing the wear between the mating articular surfaces. Mating surfaces may conform to such an extent that biological fluids, which would normally provide joint lubrication, are expressed from the PIP prosthetic joint, and the resulting xe2x80x9cdry jointxe2x80x9d may experience increased friction between the congruent articulating surfaces. Thus, it is now felt that a prosthetic PIP joint should preferably avoid the use of substantially congruent articular surfaces.
Still another important design consideration for prosthetic PIP joints is preserving the structure and function of the ligamentous tissues which surround the joint. The collateral ligaments which run along each lateral side of the PIP joint comprise both fan-like collateral ligaments and cord-like collateral ligaments (see FIGS. 2a and 2b). The fan-like collateral ligaments insert at both sides of the distal portion of the proximal phalanx and serve to support the volar plate, thereby forming an important part of the overall PIP joint. The cord-like collateral ligaments, which also insert at both sides of the distal portion of the proximal phalanx, serve to resist subluxation-dislocation of the middle phalangeal bone; because they insert at about the center of rotation of the PIP joint, they do not undergo any dramatic change in tension during flexion/extension of the PIP joint. A prosthetic PIP joint should be designed so as to preferably not require the excision of the attachment sites or disruption of the collateral ligaments and/or the volar or palmar plate. Additionally, a prosthetic PIP joint should preferably not alter the manner in which the collateral ligaments and/or the volar plate wrap around the joint during PIP joint flexion and extension.
The retinacular ligaments cross each lateral side of the PIP joint as seen in FIGS. 2c and 2d. On flexing the DIP joint, each retinacular ligament becomes taut and pulls the PIP joint into flexion. Similarly, on extending the proximal joint, the distal joint is pulled by the retinacular ligament into nearly complete extension. A prosthetic joint should preferably not interfere with the function of the retinacular ligaments.
Yet another important design consideration for a prosthetic PIP joint is the preservation of the structure and function of the tendinous tissues which surround and motivate the PIP joint. The PIP joint can only flex and extend for it has just one degree of freedom. The extensor expansion is the extensor mechanism of the finger. The tendons of extensor digitorum form the extensor expansions of all fingers, see FIGS. 2a and 2b. The extensor digiti minimi tendon contributes to the extensor expansion of the little finger. The extensor indicis tendon contributes to the extensor expansion of the index finger. The lumbricals and interossei also contribute to the extensor expansions of the fingers. The tendons of flexor digitorum profundus (FDP) and flexsor digitorum superficialis (FDS) form the flexor mechanism of a finger. The FDS tendon crosses the bottom of the PIP joint and attaches to the proximal volar region of the middle phalanx. The FDP tendon crosses the bottoms of both the PIP and DIP joints to attach to proximal volar surface of the distal phalanx.
For any finger, the median band, also referred to as the xe2x80x9ccentral slipxe2x80x9d, of the extensor expansion crosses the top of the PIP joint and attaches to the proximal dorsal surface of the middle phalanx. The median band of the extensor expansion wraps around the distal head of the proximal phalanx as the PIP joint is flexed from full extension to full flexion which, for the PIP joint, is approximately 100 degrees. The distal head of the proximal phalanx is bicondylar, as is the distal head of the middle phalanx, and the median band tracks between the condyles of the distal head in an intercondylar notch. The volar surface of the distal portion of the median band has a distinct region which mates with the intercondylar notch of the distal head of the proximal phalanx and which Professor John Stanley has recently named the Intercondylar Centering Pad (ICP). The ICP essentially fills the intercondylar notch, centering the median band of the extensor expansion between the condyles of the distal head of the proximal phalanx. The ICP enhances the lateral stability of the median band of extensor expansion as it wraps around the PIP joint during flexion/extension. If the median band were to slide laterally during extension of the PIP joint, the moment arm of the median band, i.e. the distance the tendon is from the center of rotation of the joint, would shorten which in turn would decrease the amount of extension torque applied to the PIP joint. Such a situation would likely result in an extension lag, i.e. an inability to fully extend the PIP joint.
The median band of the extensor expansion is taut during flexion of the PIP joint (see FIG. 2d) because of the passive elastic properties of the extensor muscles. If the median band were to slide laterally during flexion of the PIP joint, the volar subluxation/dislocation force created by the pull of the FDS and FDP tendons would only be resisted by the collateral ligaments and the articular surfaces of the PIP joint. Such a situation would increase the likelihood of volar subluxation-dislocation of the middle phalanx. A prosthetic PIP joint should preferably not alter the manner in which the median band and ICP of the extensor expansion wrap around the PIP joint during PIP joint flexion and extension.
The lateral bands of the extensor expansion cross each lateral side of the PIP joint, passing distally and wrapping dorsally to unite along the dorsal surface of the middle phalanx. The united lateral bands the cross the top of the DIP joint and attach to the proximal dorsal surface of the distal phalanx. During flexion of the PIP joint, the lateral bands (as they cross each lateral side of the PIP joint) slide from positions above the center of rotation of the PIP joint to positions below the center of rotation of the PIP joint. Proper prosthetic PIP joint design should have concern for the function of the lateral bands of the extensor expansion and allow the lateral bands to move normally during flexion-extension.
An implant device is provided for replacement of the complete PIP joint of the human finger in the form of a prosthesis composed of two complementary elements or members. One element, referred to as the PP (proximal phalangeal) element, replaces the distal articular portion of the proximal phalangeal bone, and the other element, referred to as the MP (middle phalangeal) element, replaces the proximal articular portion of the middle phalangeal bone. The PP element terminates in a generally bycondylar convex surface that articulates with a complementary biconcave surface formed at the proximal end of the MP element. By xe2x80x9cbiconcavexe2x80x9d surface is meant a surface that has two side-by-side shallow smooth cavities having a curvature that is complementary to but not congruent with the convex surfaces of the two condyles. The implant device is provided in a limited range of sizes of PP elements and MP elements. To accommodate the expected variation in patient anatomy, design and dimensioning are carefully controlled so that the biconcave articular surface of a given size of a MP element will smoothly receive and mate with the convex articular condylar surfaces of the PP elements of at least two, but usually three, different sizes thereof, and vice versa. Also provided is an improved method for replacing a deteriorated interphalangeal joint.
The PP element""s articular head has distal or a front bicondylar convex articular surface and a proximal or rear surface which will abut the bone in the form of two intersecting planes that extend to the dorsal and volar boundaries of the bicondylar convex articular surface. The convex bicondylar distal articular head of the PP element member has lateral planar sides which are parallel to the longitudinal axis thereof, but are sloped so that the width (longitudinally) of the dorsal region of the head is less than the width of the volar region of the head. These sloped planar lateral sides of the proximal head provide substantially free paths for the fan-like and cord-like collateral ligaments, the lateral bands of the extensor expanson and the retinacular ligaments which run along each lateral side of the of the PIP joint. The two planes that define the proximal surface of the PP element intersect along a line that lies volar of the longitudinal axis of the PP element and distal and volar to the center of rotation of the PIP joint; thus, implantation does not disrupt the attachment sites of the collateral ligaments or the attachment site of the volar plate. This design wherein the lateral dimension of the dorsal portion of the bicondylar distal head of the PIP element is smaller than the lateral dimension of its volar portion allows the lateral bands of the extensor expansion to move normally during flexion/extension. The volar plane of this proximal surface lies at an angle of less than 45xc2x0 from the long axis of the implant, preferably about 30xc2x0, which orientation minimizes the extraction force created by eccentric joint reaction forces.
The MP element""s proximal articular head has a generally elliptical outline, except for a relief means on its dorsal aspect, and it has a biconcave articular surface. This dorsal relief means accommodates the ICP and assures that the motivating and stabilizing function of the median band is maintained. The head is shaped to be thinner along its volar aspect so as not to disrupt the attachment sites of the collateral ligaments, to avoid interference with the volar plate during flexion and to provide a free path for the collateral ligaments. The relief means on the dorsal aspect of the MP element is proportioned to provide a free path for the median band of the extensor expansion to wrap around the articular head of the adjacent PP element. Furthermore, because the median band has a free path, the ICP of the median band will fill the intercondylar notch of the proximal phalangeal member and laterally stabilize the median band during flexion-extension. Also, the MP element""s head has two dorsal protrusions that are located at a boundary of the biconcave articular surface and shaped to generally extend over the condyles of the articular surface of the head of the PP element a sufficient distance so as to resist subluxation-dislocation of the middle phalangeal bone in the volar direction when the finger is in extension.
The improved prosthetic PIP joint realizes the aforementioned objects, features and advantages in a manner that is clearly evident from a thorough consideration of the detailed description when taken in conjunction with the drawings wherein there is shown and described certain preferred embodiments incorporating various features of the invention.